1. Field of the Invention
The present invention relates to an electric power conversion device including an IGBT, and more particularly to an electric power conversion device including a current conversion device requiring highly accurate current measurement.
2. Description of the Related Art
As a next-generation car comprised of a combination of an engine and a motor, hybrid automobiles are attracting attention. As the motor for use in hybrid automobiles, there has been employed a synchronous motor being small in size and capable of generating a high torque, and having embedded therein a permanent magnet. To extract the torque of the synchronous motor to a maximum extent, vector control is generally used. FIG. 8 is an exemplary control block diagram of such vector control. Current commands are calculated according to speed ω and torque command τr generated by accelerator or brake command. Based on three-phase output current and magnetic pole position θ detected by a magnetic pole position detector 18, the three-phase current is coordinate-converted to d-q coordinates to produce iq and id. Current control signals Vqr and Vdr are generated according to iq and id and current commands idr and iqr. Further, re-conversion from d-q coordinates to three phases is performed, and a driver IC 50 generates a PWM signal and drives an insulated gate bipolar transistor (hereinafter referred to as an IGBT) 19 of an inverter. At this time, dead-time error compensation is performed on the signal which has been subjected to the re-conversion from d-q coordinates to three phases.
For such vector control, a current sensor 17 for measuring an output current of the inverter is essential. In such current sensor, there are provided a core (not shown), a hall element and an amplifier; a gap is provided in part of the core disposed around the current bus, and the magnetic field generated by the current to be detected is detected by the hall element disposed within the gap and converted to a voltage.
In Japanese Patent Publication No. 10-32476 (descriptions in FIGS. 1 to 4), there is described a technique in which as current detecting means not using a current sensor, a sense section is provided separately from a section having flowing therethrough the principal current of an IGBT 19 or power MOSFET, whereby the current is detected.
FIG. 9 shows an exemplary cross sectional structure of an IGBT having a sense section. A plurality of p layers 103A and n+ layers 104A which are sections having flowing therethrough the principal current, are connected by a principal emitter electrode 120. The sections (p layers 103B and n+ layers 104B) having flowing therethrough a sense current are connected to a sense emitter electrode 121. Though not shown, gate electrodes 111A and 111B are all connected. The current flowing through the IGBT is proportional to the width (gate width) of n+ layers 104A and 104B. Accordingly, when a current (sense current) of the sense section is measured, a principal current can be measured based on the ratio between the gate width of n+ layer 104B of the sense section and that of n+ layer 104A of the principal emitter.
FIG. 10 shows an exemplary circuit for measuring a principal current according to the sense current. A voltage generated in a resistor Rs by sense current Is is measured by a voltage detector 16, and principal current Im is measured by multiplying sense current Is by the gate width ratio of n+ layer 104A of the principal emitter. In the present circuit, when the voltage detected by the voltage detector is a set voltage or more, it is determined that an excess current status occurs, thus cutting off the IGBT.
In the above described technique using a current sensor, the current sensor, composed of plural components, is costly. Furthermore, a magnetic core is used in the current sensor, so the magnetic force is lost at high temperature, and thus the current sensor cannot be used at high temperature.
FIG. 11 shows output characteristics of IGBT and power MOSFET. In power MOSFET, current is proportional to voltage ranging from 0 V. In contrast, in IGBT, current hardly flows when voltage is low, and current sharply increases at a certain voltage value or more. Also, in power MOSFET, current decreases in inverse proportion to temperature. In contrast, in IGBT, the voltage value at which current begins to rise, lowers as temperature rises, but once current rises, the slope of current relative to voltage becomes smaller. In this way, the voltage-current characteristics of IGBT are non-linear and vary in a nonlinear manner relative to temperature, so it is difficult to accurately measure a principal current according to the sense current.
Accordingly, an object of the present invention is to provide an electric power converter including means for detecting a principal current of IGBT with high accuracy.